The invention relates to a device for optical splitting and modulation of electromagnetic radiation, in particular monochromatic coherent radiation, in particular light beams and/or laser beams.
Such devices are employed for optical splitting of beams, in particular light beams and/or laser beams, for instance as optical switches or optical modulators, such as primarily in laser printers or lithography systems. Such devices, in which a beam and/or light, e.g. a laser beam, is split into a plurality of beams using an acousto-optic element, the acousto-optic element being controlled by a plurality of electrical signals having different frequencies in order to divide the beam into different beams, make it possible, e.g. when employing laser printers or lithography machines, to influence a plurality of laser beams, both in terms of their energy and their position, which means a substantial advantage in terms of speed compared to conventional machines that work with only a single laser beam. Despite this, a number of problems occur when using an acousto-optic element for generating a plurality of laser beams. One of the biggest problems is that the energy of the beams diffracted in an acousto-optic element varies with the number of beams produced and thus with the number of the frequencies applied to the acousto-optic element, which leads to a change in the individual beam energies. In addition, a temporal change in the beam energies occurs in that, when using a plurality of frequencies that are fed into an acousto-optic element simultaneously, beats occur between the individual frequencies, which leads to periodic energy fluctuations in the individual beams. The quality of the machines suffers significant negative effects due to these phenomena.
Thus for instance the problem can occur that the amplitudes of an electrical signal do not change linearly in an acousto-optic element and in addition periodic changes in the signal amplitudes occur due to beat effects in the individual frequencies. This leads to great complexity in terms of the accuracy and the speed of amplitude control, which makes it nearly impossible to perform in situ control of the beam energies via signal amplitudes. In addition, non-linear optical and electronic effects that occur due to the superimposition of the individual frequencies lead to the occurrence of higher-order beat frequencies that generate additional diffracted laser beams and that interfere with the writing and exposure processes in a laser printer or laser lithography system.
Known from U.S. Pat. No. 5,890,789 is such a device that contains as the radiation source of monochromatic coherent electromagnetic radiation a laser source, the light or laser beam of which is split into a plurality of beams using an acousto-optic element, the acousto-optic element being controlled by a plurality of electrical signals having different frequencies in order to divide or split the laser beam into different beams. For this, a signal generator, an image processor, a modulation circuit, and an intensity-control circuit are provided in order, first, to make it possible to split the laser beam into a plurality of beams, and second, to keep constant the energy of the beams produced. The intensity-control circuit controls the light intensity of the laser source as a function of the number of beams to be produced and ensures that the light intensity of the laser source is proportional to the number of beams produced so that their intensities can be kept constant, regardless of the number of laser beams produced. By keeping the intensities of the individual laser beams constant by regulating the laser source, it is supposed to be possible to keep constant the amplitude signals of the electrical signals for the aforesaid signal generator and thus to prevent changes in the beat behavior of the acousto-optic system due to a change in amplitude. However, in the aforesaid device the problem occurs that, for one thing, the regulation of the laser source at a high speed, as is required for instance in commercial laser lithography systems, is only possible for very few types of lasers, and therefore the employability of the device is very limited. For another thing, the effect explained in the foregoing is retained that, higher-order beat frequencies occur in the acousto-optic element due to superimposition of the individual frequencies and produce additional diffracted laser beams that influence laser energy periodically and interfere with the writing and exposure processes, for instance in a laser printer or laser lithography system.
In one alternative embodiment in U.S. Pat. No. 5,890,789, a mechanical shutting member is provided, by means of which a plurality of successive beams can be shut out in order to split the laser beam into a plurality of beams and to keep constant the energy of the beams produced. In this case, regardless of the number of beams that are used for writing a data set, for instance in a laser printer or laser lithography system, the number of frequencies applied to the acousto-optic element is to be kept constant using the shutting member to shut out beams that are not used and thereby the intensities of the beams used are to be kept constant regardless of the number of laser beams produced. However, this leads to other problems that interfere with the writing and exposure processes, for instance in a laser printer or laser lithography system. Due to the higher number of frequencies used, the beat phenomena described in the foregoing are amplified and, due to the increased number of combination possibilities for the different frequencies, even compounded. What this leads to is that the intensities of the individual laser beams do not remain periodically constant across the temporal course of the scan and thus it is not possible to ensure constant uniform energy distribution for the beams that have been split off.
Furthermore, the problem can occur that even slight differences between the laser source and an additional beam source, e.g. in terms of wavelength, lead to a difference in terms of the Bragg angle of the diffracted beam, which leads to a shift in the focal point for the laser beam and interferes with the writing and exposure processes in a laser printer or laser lithography system.
Even if a correction signal is calculated as a function of the number of laser beams used, the effect is retained that, due to the superimposition of the individual frequencies, beat phenomena occur that produce a temporal energy fluctuation regardless of the number of laser beams used and thus interfere with the writing and exposure processes in a laser printer or laser lithography system.